We have shown that nocturnal asthma (NA) is associated with increased parenchymal inflammation; however, whether the inflammation results in a functional change in the distal lung is not known. Therefore, we studied 10 subjects with NA, 4 subjects with non-nocturnal asthma (NNA) and 4 nonasthmatic controls (C). They underwent lung function testing followed by esophageal balloon placement and bronchoscopy in a random order at 4 pm and 4 am, respectively. The bronchoscope was wedged in the anterior segment of the right lower lobe followed by insertion of a size 5 French catheter. Warm, saturated air at flows ranging from 100 ml/min-1000 ml/min exited the catheter through one lumen and pressure was measured through the second lumen. Resistance (Rp) was defined as the pressure/flow averaged over three or more flow levels for each subject. Flow was stopped at functional residual capacity and the pressure was allowed to decay for 10 seconds; the time to decay by 63% (1/e) of the total was defined at the time constant (TC). Compliance (Compl) was calculated as TC/Rp. The Rp was significantly higher in NA (p=0.002 at 4 am and 0.003 at 4 pm) and compliance was significantly lower (p=0.02 at 4 am and 0.003 at 4 pm) than NNA and C (see table). Group Rp 4 am Rp 4pm Compl 4 am Compl 4 pm (cm H20/ml/min) (cm H20/ml/min) (ml/cm H20) (ml/cm H20) C 0.009 [0.006-0.011] 0.011 [0.001-0.013] 454 [205-705] 280 [141-522] NNA 0.035 [0.022-0.048] 0.035 [0.022-0.048] 57 [21-154] 71 [32-126] NA 0.124 [0.055-0.165] 0.113 [0.059-0.200] 10 [5.4-14.6] 8 [4-19] In addition, when related to whole lung function, the residual volume correlated with the Rp at both 4 am and 4 pm (4 am: r=0.60, p=0.03; 4 pm: 0.71, p=0.004). On the other hand, FEV1 and FVC did not correlate with the Rp at either time point. We conclude that these findings are consistent with the hypothesis that, in asthma, increased inflammatory activity causes progressive parenchymal dysfunction.